Patent Application
20130316085
The present application claims priority under 35 U.S.C.§119 of European Patent Application No. 12169320.4 filed on May 24, 2012, the disclosure of which is expressly incorporated by reference herein in its entirety.
Not a applicable.
Not applicable,
Surface finishing is understood as the most varied technical methods to enhance or modify the surface properties of substrates. Substantially, in this respect, the functional and decorative properties of the surface of a substrate are modified, with functional properties meaning the enhancement of surfaces, for example with respect to corrosion protection or wear protection, or decorative properties meaning, for example, the gloss level or the color. Known examples for surface-finished products are plastic parts or metal parts, for example DVDs or tools whose scratch resistance is improved or the manufacture of dirt-repellent surfaces on glass and ceramics (lotus effect),
Two process classes used for surface finishing are coating processes and the modification of a layer region in the substrate. The main methods of the first class, that is the coating processes, are e.g. physical vapor deposition, chemical vapor deposition and thermal spraying, in particular plasma spraying, but also classical methods such as lacquering. The term physical vapor deposition (PVD) in this respect designates a group of vacuum-based coating processes or thin film technologies. In this respect, the coating material is transformed into the gas phase using physical processes and the gaseous material is condensed on the substrate, with the target film being formed.
In chemical vapor deposition (CVD), in contrast, the gas phase of a solid component is frequently deposited on the heated surface of a substrate due to a chemical reaction. The method is usually characterized in this respect by a chemical reaction in the process chamber or at the surface of the workpiece to be coated. The frequently high temperature load on the substrate represents a substantial limitation. The thermal load can cause, among other things, distortion at workpieces or it can lie above the softening temperature of the material to be coated so that the method cannot be used or can only be used with restrictions.
In a further category of coating processes, thermal spraying, additional materials, the so-called spray additives, are melted off, partially melted or fully melted inside or outside a spray torch, are accelerated in the form of spray particles in a gas flow and are bombarded onto the surface of the component to be coated. A layer formation takes place since the spray particles flatten more or less in dependence on the process and on the material on an impact onto the component surface, adhere primarily due to mechanical fusion and so build up the spray layer.
The units used for the different methods, for example vacuum deposition plant, sputtering plant, plant for CVD and thermal spraying plant, for example therma plasma spraying units, are today used in a number of areas of industrial production.
Typical substances include, for example, workpieces having curved or non-curved surfaces, for example tools or cylinder running surfaces of internal combustion engines, a number of components and semi-finished products on which a corrosion protection is, for example, applied by means of a thermal spraying method, but also planar substrates such as wafers and films onto which a coating is applied, that is inter alia conductive or insulating layers for semiconductors such as solar cells. The applied layers can be used to make the surface resistant toward heat, mechanical, chemical or corrosive influences, to improve friction or adhesion on the surface, to make the surface electrically and/or thermally insulating or conductive, from case to case, to make the surface compatible for foodstuffs or for blood or tissue or to form seals and diffusion barriers, to name just a few typical applications.
The most varied plant and processes are already known from the prior art for the reactive pretreatment of the outer surface, that is for example, for the activation of the surface prior to a coating process, and for depositing thin films by means of a plasma. A thermal spraying process using plasma is described in EP 2 107 131 A1. The process described there deals with a method for the coating and/or for the surface treatment of substrates by a plasma jet which is produced in a process chamber by means of a plasma source. In this respect, plasma gas is conducted through the plasma source, the plasma gas is heated by means of electrical gas discharge and/or electromagnetic induction and/or microwaves and the plasma jet is directed onto a substrate introduced into the process chamber.
The process is characterized in that a reactive component is injected into the plasma jet in liquid or gaseous form to coat a surface of the substrate or to pretreat the outer surface prior to the coating, that is to activate it, to achieve a better adhesion of the coating. The reactive component is in this respect supplied to the plasma source in gaseous or liquid form. Possible activation processes to improve the adhesion of the coating on the surface of the substrate include, for example, the heating, cleaning, etching, partial oxidation or partial nitration of the outer surface, e.g. by means of a plasma jet.
Substantial disadvantages of the process described in EP 2 107 131 A1 are that the pretreatment, that is the activation, of the surface, and the coating of the surface are very time-consuming.
If, in contrast, the substrate as such is to be modified beneath or deep in the surface, i.e. in a boundary region beneath the surfaces, diffusion processes well-known from the prior art are known such as nitriding, carburizing, nitrocarburizing, oxide nitriding, carbonizing, oxidizing or borizing. Diffusion methods are processes in which a layer region in the substrate is modified, usually at elevated temperatures, that is e.g. by diffusing of, frequently, nitrogen or carbon, or other reactive substances into the substrate surface, a modified layer is formed beneath the surface of the substrate. A disadvantage in this respect is that the required process times for the treatment of the substrate are very long, up to 100 hours in part, which is uneconomic and only of limited suitability for many industrial applications.
It can thus be stated in summary that the two discussed classes of methods admittedly provide possibilities for coating at the surface of a substrate or for modification by diffusion in a layer region beneath the surface of the substrate, but have the mentioned disadvantages for an efficient production on an industrial scale.
It is therefore the object of the invention to provide a new diffusion process in which the disadvantages of the know processes and methods are avoided and to propose a diffusion method which is less time-consuming and thus more economic than, for example, classical nitriding, carburizing or nitrocarburizing.
This object is satisfied by the method defined in claim 1.
The dependent claims relate to particularly advantageous embodiments of the invention.
The invention thus relates to a method of modifying a boundary region of a substrate bounded by a surface, wherein an evacuated process chamber is provided having a plasma source for producing a directed plasma jet, and wherein furthermore a reactive component is supplied into the process chamber with a flow of predefined size and the substrate is heated to a predefined reaction temperature. The reactive component is diffusion-activated in accordance with the invention by the directed plasma jet so that the reactive component diffuses into the boundary region of the substrate at a predefined diffusion rate.
It is thus important for the invention that the reactive component is diffusion-activated by the directed plasma jet such that the reactive component is present in a very high concentration, in particular in the region of the plasma jet, and diffuses into the boundary region of the substrate at a predefined diffusion rate. Diffusion activation is to be understood as a direct breaking or splitting open of the reactive components, for example into individual atoms or ions. It is particularly advantageous in this respect that the diffusion activation takes place, in comparison with known processes, in the region of the plasma jet and at a high temperature so that a high concentration of reactive components arises, in particular in the region of the plasma jet. Due to the high local concentration of the reactive component in the region of the plasma jet, but also in the marginal region of the plasma jet or in the process chamber outside the plasma jet, an improved diffusion behavior occurs, i.e. the reactive component diffuses faster into the boundary region of the substrate. The process times are thus reduced by a multiple and the invention makes it possible to employ the present method in industrial processes and to utilize it economically.
In contrast to the known method described in EP 1 160 224 B2, in which an arc is ignited at a lower voltage, usually DC current, in a plasma reactor, and the reactive component is distributed uniformly in the process space, the invention utilizes a modified process of low pressure plasma spraying (LPPS) using a directed plasma jet to bring about a directed diffusion process at specific boundary regions of the substrate. The diffusion dynamics are thus decisively improved by the present invention because a direct increase in the concentrations at reactive components, in a predefined diffusion rate and predefined process parameters, is achieved or improved by the use of the modified LPPS principle.
In practice, the reactive component is liquid and/or gaseous and/or powdery and/or a suspension, with the reactive component for diffusion activation preferably being injected into the plasma jet of the plasma source and/or into the process chamber. If the reactive component is present as a suspension or in liquid or gaseous form, three preferred embodiment variants are advantageously possible, either the injection of a reactive component into the plasma jet in the plasma source or into the free plasma jet or into the process chamber.
Depending on the embodiment variant, an injector is provided in the plasma source to inject a reactive component into the plasma jet. The injector can be arranged in the region of a nozzle which is provided for forming the plasma jet in the plasma source. The reactive component can, however, also be injected into the free plasma jet by means of an injector, for example by means of an injector which is arranged at a specific distance from the nozzle outlet opening of the plasma source or by means of an injector which injects the reactive component into the process chamber or is otherwise introduced into the process chamber and/or into the plasma jet. As long as the plasma jet is not fanned out a lot, the injector is advantageously arranged substantially centrally in the plasma jet. If the plasma jet is fanned out more, for example at a distance of typically more than 0.1 m from the plasma source, ring-shaped injectors can, for example, also be provided.
If the reactive component is present as a powdery reagent, in a further advantageous embodiment, the plasma source is provided with one or more infeeds to supply these powdery solid particles, but also a suspension, and thus to modify the boundary regions of the substrate by means of the modified LPPS process in accordance with the invention.
The infeed can, for example, be conducted up to and into the region of a nozzle which is provided for forming the plasma jet in the plasma source to introduce reactive additives in the form of liquid or gaseous reactive elements or as powdery solid particles and/or as suspensions into the plasma jet at this point. The powdery solid particles are in this respect advantageously supplied by means of a conveying gas.
The reactive component, which can include a hydrocarbon compound and/or oxygen and/or nitrogen and/or another reactive substance, diffuses through the surface into the boundary region of the substrate and in so doing a compound, in particular a nitride or a nitro compound, a carbide, a nitrocarbide, a silicide, a carbonitride, an oxide, a boride or an aluminide is created.
Possible modifications of the boundary region of the substrate thus include nitriding, carburizing, aluminizing, nitrocarburizing, oxide nitriding, carbonitirding, oxidizing or borizing the substrate. The decisive difference from coating the surface of the substrate is in this respect that actually not coating is produced which is sufficiently known from the prior art, but rather a reactive component is diffusion-activated and a boundary region in the substrate is modified by diffusion processes by means of a modified LPPS process. Some embodiments of modification processes which were manufactured using the above-described method will be explained in more detail in the following.
As a specific advantageous measure, the process chamber includes a heat source to be able to carry out the modification at a reaction temperature within a predefined temperature range, for example to be able to directly control the diffusion. As a further measure, the substrate can be preheated to a reaction temperature by means of the additional heat source and/or the reaction temperature can additionally or alternatively be controlled or regulated by means of the plasma jet during the modification.
Since the substrate can be preheated before the modification; for example, in dependence on the embodiment variant, a heat source can be advantageously provided for the process chamber or in the process chamber for the substrate. The process parameters such as the diffusion rate or the diffusion speed, etc. can thus be ideally set prior to the modification and to improve the modification of the boundary layer, that is to achieve an economically efficient process and to achieve short modification and/or process times. The preheating of the substrate can in this respect take place using the same process parameters as the modification of the substrate, with it normally being sufficient to lead the plasma jet, possibly not containing any reactive component for preheating, over the substrate e.g. with a few pivot movements. In the ideal case, the heat source is not required or is only required for a short period and the plasma jet contains the reactive component so that just a few pivot movements are sufficient to heat the substrate surface to a required reaction temperature. An infrared lamp, a hot plate, the plasma jet itself or any other suitable heat source can be used as an additional heat source, for example. The temperature monitoring can be carried out with common measuring processes, e.g. using infrared sensors, thermal sensors, etc.
A particularly advantageous measure provides that hydrogen is supplied to the process chamber, in particular to the plasma jet. The hydrogen is in this respect supplied either into the plasma jet or into the process chamber. The supply of hydrogen has two advantages. On the one hand, the hydrogen serves as a catalyst for the diffusion process; on the other hand, the hydrogen prevents the reoxidation of the substrate, for example if the substrate is a zirconium oxide or another oxide. The function as a catalyst results in that an additional energy supply takes place in the boundary region of the substrate, in particular on the interface, by the recombination of free hydrogen ions and the diffusion or the modification process is thus assisted.
As a further measure which optimizes the modification or the diffusion, the reaction temperature of the substrate is set to a value in the range from 500-1200° C., in particular 800-1100° C. The setting and maintaining of the reaction temperature in a predefined temperature interval is necessary because the quality and the process speed can thus be fixed and influenced. It is moreover advantageous in order, for example, to be able to ensure a uniform quality of the modification, that the substrate has a temperature distribution which is as homogeneous as possible, which can be realized, for example, by the heat source or by the plasma jet. Due to the increased temperatures, the diffusion depth of the reactive component into the boundary layer of the substrate is improved so that a particular advantage of the method in accordance with the invention is a deeper penetration into the substrate and a greater thickness of the modified boundary layer resulting therefrom.
It has proved to be particularly advantageous that the method is particularly efficient when the plasma source and the substrate holder are moved relative to one another. It is furthermore advantageous if the substrate is held by a substrate holder, in particular by a tantalum wire. After the preheating of the substrate, in particular of the boundary surface, the modification of the boundary layer bounded by the surface is started, with the plasma jet, which contains the reactive component in dependence on the embodiment, preferably, but not necessarily being led over the substrate by means of pivot movements. From case to case, in addition to or instead of pivoting the plasma jet, the substrate can be moved by means of the substrate holder or the substrate is moved by rotary or pivot movements relative to the plasma jet during the modification process. Al the named measures have the advantage that a uniform treatment or coating is thereby achieved and possible local hot spots and/or damage to the substrate surface, or to the substrate, are avoided which may arise with a constantly aligned plasma jet with a high radiation power.
In another advantageous embodiment, the tantalum wire is used for fixing and holding the substrate so that the plasma and the reactive component completely cover the substrate and a homogeneous modified boundary layer is created, with the tantalum wire not reacting with the reactive component and being particularly suitable for use in an industrial production process.
It is furthermore advantageous if a controlled adjustment apparatus is provided for the plasma source to control the direction of the plasma jet and/or the distance of the plasma source from the substrate, e.g. in a range from 0.05 m to 1 m, in particular in a range from 0.3 m to 0.6 m. When the plasma source and thus the plasma jet are led over the substrate by means of pivot movements, that is moved with rotary or pivot movements relative to the substrate during the modification process, a uniform treatment and modification of the boundary region is thereby achieved and possible local hot spots and/or damage to the substrate surface or to the substrate are avoided,
The distance of the plasma source from the substrate can amount, for example, to 0.3 m-0.6 m, the pressure in the process chamber to 0.2 mbar to 1 mbar and the power which is supplied to the plasma source lies between 1 kW and 100 kW, in particular between 1 kW and 40 kW. The pressure in the process chamber during the process amounts to between 0.01 mbar and 100 mbar, in particular to between 0.01 mbar and 10 mar. In addition, the gas quantity flow of the reactive component during the process amounts to between 1 SLPM and 100 SLPM, in particular between 1 SLPM and 10 SLPM for nitrogen or between 0.1 SLPM and 2 SLPM for methane. It becomes possible by a suitable choice of the parameters, e.g. the aforesaid parameters for preferred embodiments, to manufacture high-quality modified boundary regions in the treated substrates and to accelerate the total process so advantageously that an efficient use in the industrial sector is possible.
A main point of the method of the invention is thus the design of a modified LPPS process in the form of a diffusion method. It can thus be stated in summary that with the method presented of modifying a boundary region of the substrate, which is bounded by the surface of the substrate, the desired optical, physical and mechanical properties of the substrate can be efficiently manufactured as an industrial production method.
A specific embodiment will be presented again in the following which again illustrates a specific possible production method and which has particular importance for practice. In this respect, the individual method steps will again be briefly presented and explained with reference to an embodiment which provides the decorative modification of the boundary region of a ZrO2 substrate with carbon C. The method in accordance with the invention described in the following is in this respect in particular suitable for treating bracelets, for example for wristwatches, in order to give them, among other things, a desired decorative appearance and color.
The following parameters are used in a specific embodiment of a method in accordance with the invention, wherein the parameters or parameter ranges used are indicated directly after the parameter, whereas further possible parameters or parameter ranges are additionally indicated in parentheses.
If the method is used for the decorative modification of a substrate, e.g. of a bracelet or of a timepiece casing, the substrate, for example a metal or a ceramic material, can possibly be advantageously polished before the modification so that the substrate has e.g. a metallic appearance, for example gold colored, after the modification if the boundary region is nitrided or dark gray, for example, in the case of carburizing. Depending on the composition of the plasma jet, of the reactive component, of the position of the substrate, etc., there are no limits to the color section here, for example blue, red, etc. Furthermore, other properties of the surface such as the scratch resistance, hardness, etc. can also be set directly by the use of a method in accordance with the invention.
For fixing, the substrate is fastened to a substrate holder, preferably by means of a tantalum wire, so that the plasma covers the whole substrate and the latter is uniformly modified. The tantalum wire in this respect usually does not react with the reactive component so that the latter can be used over and over again. In addition, the substrate holder can be designed such that the latter, unlike the known classical processes in which some few workpieces are arranged in a circle., can be used for several hundred workpieces. In addition, the plasma source and the substrate are moved relative to one another to coat all workpieces uniformly, with the reaction temperature preferably being regulated or controlled.
The heating of the substrate is an important part of the method since the quality and speed of the diffusion process are defined thereby. It is important in this respect that the substrate has a temperature distribution which is as homogeneous as possible and which can lie between 800 and 1100° C. to allow an ideal diffusion process. The addition of hydrogen to the plasma gas in this respect additionally increases the energy density in the boundary region and on the surface of the substrate. Due to the elevated temperatures, the hydrogen is split into individual hydrogen ions which recombine to H2 again, wherein the process of recombination usually takes place in the boundary region and at the surface of the substrate, whereby the modification process can be further improved.
After the preheating of the substrate, the actual modification process is started. For this purpose, the reactive component, for example methane CH4 or nitrogen N2 or another gas, liquid, suspension or solid is, for example, injected into the plasma source. The reactive component is split due to the energy of the plasma jet so that the required atoms, for example carbon C, nitrogen N or others are formed and can diffuse into the boundary region of the substrate. The modification, that is the diffusion per se, takes a few minutes (approximately 30 minutes), with a process also being possible including a plurality of steps (e.g. approx. 2×15 minutes), which further improves the diffusion process. The process in accordance with the invention is thus approximately 6 times faster than all known classical diffusion processes such as the classical nitriding, which often require 3 hours or more.
In a particularly preferred embodiment, the reactive component carbon C, which arises by injection of methane into the plasma source, is diffused into the crystal structure of the boundary region of the zirconium dioxide ZrO2 and replaces the oxygen O2 at least in part, with zirconium carbide ZrC being formed. For this purpose, the CH3 molecule is first split by means of the plasma jet and the boundary region of the substrate is heated to reaction temperature and the required reaction energy is provided so that the carbon C can diffuse into the boundary region. It is understood that the reactive gas can also be activated or split e.g. by contact with the hot substrate surface or similar. Setting the correct reaction temperature is in particular important since the diffusion process only runs ideally as a rule in a specific temperature range since, if the substrate is too hot, the diffusion process does not run ideally, and can e.g. at least partly be suppressed or reversed or, if the substrate is too cold, the reactive component, that is carbon, nitrogen or another reactive component, cannot diffuse deeply enough into the boundary region.
As already described, the CH4 is first injected into the plasma source, for example into a nozzle of the plasma source. The methane can therefore also be injected or otherwise introduced into the plasma jet or into the plasma chamber, with the reactive component in this case having a comparatively large free mean path distance and coming into contact with the plasma jet very quickly. As soon as the molecules of the reactive component come into contact with the high-energy plasma jet, the molecule can be split and as a rule different split products, that is CH, C, H2, can be found in the plasma jet.
The substrate is heated before the actual modification process to the corresponding reaction temperature which lies for zirconium dioxide e.g. preferably between 800 and 1100° C. and this range of the reaction temperature is maintained using the plasma jet during the modification, that is during the diffusion process.
A similar process results when nitrogen N is used instead of carbon C; in this case zirconium nitride is formed which has a yellow color. The method can also be used on other substrates, for example titanium, and also many other materials.
After the modification procedure, the substrate can be cooled in a sluice and simultaneously the processing of the next workpiece can be started to further increase the productivity and efficiency of the method and to maintain the evacuated state of the process chamber. The cooling process had a duration of 15 minutes in the present embodiment.
The invention will be explained in more detail in the following with reference to the drawing. There are shown in a schematic representation:
The plasma coating plant 1 includes a process chamber 2 having a plasma source 4 for generating a plasma jet 5, a controlled pump apparatus, which is not shown in
The pressure in the process chamber 2 is set to a predefined value by means of the controlled pump apparatus, with the plasma modification plant 1 additionally including an injection apparatus (not shown) to introduce, in particular to inject, at least one reactive component into the plasma jet 5 or into the process chamber 2 in liquid and/or gaseous and/or powder form and/or as a suspension.
If required, the substrate holder 8 can be designed as a displaceable bar holder to move the substrate out of a pre-chamber through a sluice 9 into the process chamber 2. The bar holder additionally makes it possible to rotate the substrate 3, if necessary, during the modification. In practice, normally a plasma source 4 is therefore used which is usually used for thermal plasma spraying. Typically, the plasma source 4 is connected to a power supply, for example to a DC supply for a DC plasma torch, and/or to a cooling apparatus and/or to a plasma gas supply and is connected, from case to case, to a supply having the reactive component and/or a conveying apparatus for powdery reactive components or suspensions.
A conventional plasma source 4 for thermal spraying can include, for example, an anode and a cathode to generate an electrical discharge, wherein the anode and the cathode are normally cooled, for example by means of cooling water, in the power range required for thermal spraying.
A process gas, also called a plasma gas, supplied to the plasma source 4 is ionized in the electrical discharge to generate a plasma jet 5 having a temperature of up to 20,000 K. The plasma jet 5 exits the plasma source 4 at a speed of typically 200 m/s to 4000 m/s. The process gas or plasma gas can, for example, include argon, nitrogen, helium and/or hydrogen or a mixture of a noble gas with nitrogen and/or hydrogen or can be composed of one or more of these gases.
As can be seen, a modified boundary region 11 is located in a boundary region 9 of the substrate 3, which is a bracelet or a housing for a timepiece, bounded by the surface 10 of the substrate 3. The modification of the boundary region 9 is in this respect formed by means of the method in accordance with the invention, that is by means of diffusion of the reactive component into the boundary region 11 and subsequent interaction of the reactive component with the substrate 3.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12169320.4 | May 2012 | EP | regional |
